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A.2.3 Character Strings in Mex-Files

As mex-files do not make the distinction between single and double quoted strings within Octave, there is perhaps less complexity in the use of strings and character matrices in mex-files. An example of their use that parallels the demo in stringdemo.cc is given in the file mystring.c, as shown below.

#include <string.h>
#include "mex.h"

void
mexFunction (int nlhs, mxArray *plhs[],
             int nrhs, const mxArray *prhs[])
{
  mwSize m, n;
  mwIndex i, j;
  mxChar *pi, *po;

  if (nrhs != 1 || ! mxIsChar (prhs[0])
      || mxGetNumberOfDimensions (prhs[0]) > 2)
    mexErrMsgTxt ("ARG1 must be a char matrix");

  m = mxGetM (prhs[0]);
  n = mxGetN (prhs[0]);
  pi = mxGetChars (prhs[0]);
  plhs[0] = mxCreateNumericMatrix (m, n, mxCHAR_CLASS, mxREAL);
  po = mxGetChars (plhs[0]);

  for (j = 0; j < n; j++)
    for (i = 0; i < m; i++)
      po[j*m + m - 1 - i] = pi[j*m + i];
}

An example of its expected output is

mystring (["First String"; "Second String"])
⇒ s1 = Second String
        First String

Other functions in the mex interface for handling character strings are mxCreateString, mxArrayToString, and mxCreateCharMatrixFromStrings. In a mex-file, a character string is considered to be a vector rather than a matrix. This is perhaps an arbitrary distinction as the data in the mxArray for the matrix is consecutive in any case.